1. Field of the Invention:
This invention relates to the production of polyphenylene ethers.
2. Description of the Prior Art
Polyphenylene ethers and processes for their production are known. Such processes are described, for example, in Buehler "Spezialplaste" (Special Plastics) (Akademierverlag, Berlin, 1978) and in U.S. Pat. Nos. 3,306,874 and 3,306,875.
In industry, processes in which di-ortho-substituted phenols are oxidatively coupled by transition metal-amine complexes in the presence of oxygen or gas mixtures containing oxygen are used. Three process steps are required for the production of the pulverent end product, namely, quenching the polycondensation, purification of the polymer and isolation of the polyphenylene ether (PPE).
Depending on the reaction conditions in each case, quenching the polycondensation can advantageously result either by inactivation of the catalyst, separation of the catalyst or separation of the polymer. In some cases, these measures cause a purification (although incomplete) of the polymer in addition to ending the polycondensation.
The most important quenching methods used in the prior art are:
1. addition of aqueous acids,
2. addition of alkalies,
3. addition of copper ion complexing agents, and
4. addition of polar solvents for separation of the polymer.
The addition of aqueous acid solutions is probably the most widely used process for quenching polycondensations. With this process, however, acid consumption is high and amine separation is unsatisfactory (cf DE-OS No. 22 46 552, page 4, lines 1 to 8). This process has been improved by operating with extraction equipment. For example by using a countercurrent of an aqueous acid stream and optionally a raised temperature (DE-OS No. 21 05 372), but the problem remains that the process requires considerable amounts of alkali metal hydroxides to recover the amines.
Abandoned DE-OS No. 22 46 552, discloses a process in which a PPE reaction formulation containing a copper amine catalyst and the polymer in an aromatic solvent is quenched with a CO.sub.2 /water mixture and then worked up. However, in this process also, the separation of the amine from the polymer is unsatisfactory.
DE-OS No. 15 70 683 describes the addition of a 50% sodium hydroxide solution to the PPE reaction formulation. This process has not gained general acceptance because it does not reliably stop the polymerization reaction (cf DE-OS No. 24 30 130).
It is known that with chelating agents a copper catalyst can be inactivated and separated (cf DE-PS No. 15 70 683, column 15, lines 3 to 5, and DE-OS No. 26 40 147). Additionally, experimental details concerning the use of chelating agents are disclosed in DE-OS No. 23 64 319. The essential drawback of all processes that work with chelating agents is degradation of the polymer. It has been established (cf DE-OS No. 27 54 887) that polyphenylene ethers are degraded faster the higher the temperature and the longer the time elapsing between quenching the reaction and isolation of the polymer. For example, the viscosity index, which is a measurement for the average molecular weight of the polymer units, usually decreases at a temperature of 50.degree. C. by more than 0.1 dl/g, not infrequently even by 0.2 dl/g, per hour (DE-OS No. 27 54 887).
This degradation cannot be avoided even by the use of special chelating agents such as, e.g., the polyamines of the process of DE-OS No. 24 60 323.
It has been proposed to add to the polymer solutions, in addition to the chelating agent, other stabilizing agents, such as bivalent phenols and reducting agents (cf DE-OS No. 27 54 887 and DE-PS No. 26 16 746) or aromatic amines (cf DE-OS No. 27 55 937). However, this approach is not satisfactory since in some cases the additives are required in considerable concentrations, and the isolation of the polyphenylene ethers is made all the more difficult by these additional stabilizing agents.
The polycondensation of PPE can also be stopped and the polymer precipitated with the addition of alcohols, ketones or their aqueous solutions. These are generally referred to as "antisolvents" which retain the catalyst residues and diphenylquinone in solution. However, even a small amount of the catalyst remaining in the polymer product can have a negative effect on its color quality and catalyze its degradation (cf DE-OS No. 26 16 746).
It is common to all precipitation processes (cf, e.g., DE-OSS Nos. 12 65 410, 25 32 477, 25 32 478, 26 55 161 and 27 52 867) that considerable amounts of solvents must be used, worked up and redistilled. Therefore in practice it is preferred to obtain the polymers by a direct isolation process, for example, by steam separation, spray drying or hot water agglomeration. But with these processes the transition metal catalyst residues and other impurities must be previously separated as quantitatively as possible (cf DE-OS No. 24 60 323).
Finally, the PPE reaction can also be quenched by simultaneous treatment with a bivalent phenol or benzoquinone and a mild reducing agent such as, for example, H.sub.2 S or hydrazine (cf DE-PS No. 24 30 130). It can be gathered from the specification of DE-PS No. 2430 130 that the resulting polymer is not subject to molecular weight degradation within a day's time. The reducing agent by itself exhibits no action. Thus, for example, the viscosity number drops within a day from 0.61 dl/g to 0.44 dl/g when quenching with a 30-fold molar excess of hydrazine (see table I, test 7). But if the process is followed in accordance with the disclosure, and bivalent phenols or benzoquinones which are only slightly microbially degradable are added, the molecular weight of the polymer product is stabilized. This is a disturbing factor (cf DE-OS No. 27 54 887).
Moreover, it is mentioned in DE-PS No. 15 70 683 that the PPE reaction can also be stopped by precipitating the copper catalyst as an insoluble compound which is filtered off before isolation of the product. With this information, one skilled in the art might think of precipitating copper salts by introducting H.sub.2 S. Actually, under the conditions of the oxidative coupling reaction, precipitation of the copper sulfide, which in itself is slightly soluble, does not occur. It is clear from tests run by the applicants (see comparison test A) that precipitation of the copper sulfide either does not occur at all or only to a slight extent. The reason for this absence of precipitation is not known.
The applicants know of no example in the literature in which a PPE polycondensation can be stopped merely by the addition of hydrogen sulfide.
There continues to exist a strong need for a process for the production of polyphenylene ethers, in which the oxidative coupling can be stopped by the addition of only one agent and at the same time protecting the resulting polymer from molecular weight degradation. Such process should further make it possible to largely free the polymer from catalyst residues and reaction by-products, and at the same time separate the catalyst in suitable form. In particular, the transition metal and quinone components in the polymer should be reduced to a value of under about 5 ppm and 20 ppm, respectively. Moreover, it is desired to obtain a polymer with perfect color quality and which can be subjected to a direct isolation process.
This invention is based on the discovery that under the conditions of oxidative coupling reactions of di-ortho-substituted phenols, hydrogen sulfide is suitable as a precipitation agent for the transition metals used as catalyst components. This process only requires that care be taken that no oxygen is present during the quenching operation.